Give the homepage teaser a rich text editor

Replace the plain textarea for the homepage_teaser setting with the shared
TipTap rich-text editor, and render the teaser as sanitized HTML in the
portal hero's default layout.

- SettingsAdmin: teaser field now uses RichTextEditor (lazy-loaded, code-split
  like PostEditor); rich fields render in a <div> wrapper instead of <label>.
- HeroElement: text-block lines flagged `html` render sanitized HTML.
- heroLayout: the default-layout teaser line is now an HTML line.
- admin.controller: sanitize homepage_teaser against the body allowlist on save.
- theme.css: collapse the teaser's nested block margins in the hero.

Closes #48

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/src/styles/theme.css

@@ -546,6 +546,20 @@ button[disabled] {
 }
 
 /* ===== Hero canvas editor ===== */
+/* Rich-text hero line (e.g. the homepage teaser). Inherits the line's font/color
+   from its inline style; collapse the editor's outer block margins so spacing is
+   driven by the line's own marginTop rather than a nested <p>. */
+.hero-rich > :first-child {
+  margin-top: 0;
+}
+.hero-rich > :last-child {
+  margin-bottom: 0;
+}
+.hero-rich a {
+  color: inherit;
+  text-decoration: underline;
+}
+
 .hero-el-editable {
   outline: 1px dashed rgba(127, 153, 189, 0.45);
   outline-offset: 2px;